In the prior art, in order to reuse scrap iron, tin plate scrap, etc., these scrap metals had to be fed in suitable amounts into molten steel at the time of refining of the molten steel. The molten steel containing the scrap iron and the like was then refined and subjected to ingot making or continuous casting to prepare an ingot or a slab having a thickness of not less than 100 mm which was then rolled to prepare a thin sheet or the like.
Particularly in recent years, however, the amount of copper contained in scrap iron has become large. When an ingot or a slab Containing the scrap iron or tin plate scrap is hot-rolled and, if necessary, then cold-rolled to prepare a thin steel sheet having a thickness in the range of from 0.1 to 15 mm, red-shortness occurs in the ingot or cast strip in the course of the hot rolling and hot tear frequently occurs, which makes it difficult to conduct hot rolling, so that the production of the above-described thin steel sheet becomes very difficult.
The red-shortness occurs as follows. When a cast strip or the like is heated before hot rolling, since copper (Cu) and tin (Sn) are less likely to scale, they are enriched on the surface layer portion of the cast strip without being removed as a scale. The enriched Cu and Sn form a low-melting liquid film and, at the same time, are unevenly distributed at grain boundaries, which renders the grain boundaries fragile at a hot rolling temperature, so that red shortness occurs.
Further, Cu and Sn are ingredients which are difficult to remove from molten steel by refining.
Therefore, scrap iron and the like containing large amounts of Cu and Sn are blended little by little in many divided charges for use in lowered Cu and Sn concentrations.
The above method, however, presents the problem that the Cu and sn concentrations of the steel product gradually increase during a use cycle for a long period of time. Further, the control and work associated with the blending of the scrap iron little by little in many divided charges are very troublesome.
In order to solve the above problem, as described in "Tekko To Gokin Genso" (volume one), 1967, pp. 381 and 385, the addition of Ni in an amount satisfying the following formula to molten steel has been carried out in the art. EQU Ni%.gtoreq.1.6(Cu%+6Sn%)
It is considered that Ni added to the above-described molten steel co-exists in a Cu-enriched layer in the grain boundary, which is an origin of the above cracking, and serves to increase the melting point of that portion and to increase the solubility of Cu in the matrix, so that it prevents the occurrence of a liquid film.
However, for a molten metal containing large amounts of Cu and Sn, for example, 0.3 to 10% by weight of Cu and 0.03 to 0.5% by weight of Sn, the necessary Ni concentration amounts to 0.8 to 21% by weight, which is a large problem from the viewpoint of cost and also from the viewpoint of properties due to occurrence of uneven surface plating and poor descaling derived from internal oxidation.
The present invention has been made with a view to solving the above problems, and an object of the present invention is to provide a thin cast strip and a thin steel sheet having a desired thickness and no surface cracking from a molten metal comprising common carbon steel ingredients, with scrap iron and tin plate scrap containing a large amount of Cu being added thereto.
Another object of the present invention is to efficiently provide a thin cast strip and a thin steel sheet having a desired thickness and no surface cracking without conducting troublesome control and work wherein scrap iron or tin plate scrap containing a large amount of Cu is blended little by little.
A further object of the present invention is to provide a thin cast strip and a thin steel sheet having a desired thickness and no surface cracking from a molten steel comprising common carbon steel ingredients not containing Ni and, added thereto, scrap iron and tin plate scrap containing a large amount of Cu being added thereto.
A further object of the present invention is to provide common carbon steel thin cast strip and thin steel sheet which contain large amounts of Cu and Sn and have excellent mechanical properties and surface quality.